Soil can exit in nature in innumerable varieties and these materials do not lend themselves to separate into distinct categories. Proper classification of soil is an important step in connection with any foundation job because it provides the first clue to the experiences that may be anticipated during and after construction. The laboratory tests, which provide information on physical properties of soil, are known as classification tests and numerical results of such tests are known as index properties. If the classification tests are properly chosen, soil materials having similar index properties are likely to exhibit similar engineering behaviour. On the basis of some laboratory tests, it has been found that soil can be classified into groups within each of which the significant engineering properties are somewhat similar.
The index properties are of a great value to the civil engineer in that, in one hand, they provide means in the correlation of construction experience and on the other hand they form a basis for information of the correctness of the field identification of a given material. It the material is improperly identified, the index properties indicate the errors and lead to correct classification. Index properties may be divided into two general types:
1. Soil Grain Properties
2. Soil Aggregate Properties
Soil Grain Properties are the properties of the individual particles of which the soil is composed of and are independent in the manner of soil formation. These properties can be determined from distributed samples. Soil Aggregate Properties, on the other hand, depend on the structure and the arrangement of the particles in the soil mass, whereas the soil grain properties are commonly used for soil identification and classification. The soil aggregate properties have a greater influence on the engineering behaviour of soil mass. The engineering behaviour of a soil mass depends on its strength, compressibility and permeability characteristics. The most important aggregate property of a coarse grained soil is its relative density while that of a fine-grained soil is its consistency.